CSS Code Housekeeping (#3026)

* refactor: Avatar classes refactor
* refactor: Badge classes refactor
* chore: Remove commented dead code
* chore: Remove SignUpModal dead CSS code
Flarum seem to have had some kind of user display in the sign up modal 
on successful sign up, which no longer exists.

https://github.com/flarum/core/blob/v0.1.0-beta/js/forum/src/components/SignUpModal.js#L111
* chore: Deprecate unneeded vendor mixins
* chore: Normalize property values format
Co-authored-by: David Wheatley <hi@davwheat.dev>
* chore: Remove @-webkit-keyframes
* chore: Combine animation properties
* chore: Avoid `all` for transition
* chore: translate3d is no longer necessary for hardware acceleration
* fix: Lost cursor pointer to normalize update
* chore: Use CSS variables for more things
* chore: Remove unecessary overspecification
Co-authored-by: David Wheatley <hi@davwheat.dev>

less/common/Button.less

@@ -77,7 +77,7 @@
   fieldset[disabled] & {
     cursor: default;
     opacity: 0.65;
-    .box-shadow(none);
+    box-shadow: none;
   }
 
   a& {
@@ -88,7 +88,7 @@
   }
 
   .Button-label {
-    .transition(margin-right 0.1s);
+    transition: margin-right 0.1s;
   }
 
   .LoadingIndicator-container {
@@ -148,7 +148,7 @@
   &.focus,
   .open > &.Dropdown-toggle {
     background: transparent !important;
-    .box-shadow(none);
+    box-shadow: none;
     color: @link-color;
   }
 }
@@ -164,7 +164,7 @@
   &:active,
   &.active,
   .open > &.Dropdown-toggle {
-    .box-shadow(none);
+    box-shadow: none;
   }
 }
 .Button--primary {